Several parts combine into one character.

鏡 is a phonetic-semantic compound: 金 (metal) on the left provides the meaning, while 竟 (jìng, "to finish / complete") on the right provides the sound. The semantic logic is precisely historical: ancient East Asian mirrors were not glass but polished bronze disks (青銅鏡), painstakingly ground and burnished until the surface achieved sufficient reflectivity. The "metal" radical is technologically accurate to the era. Bronze mirrors recovered from Silla tombs in Korea and Kofun-era burials in Japan are physical artifacts of this character's original referent. Mainland China simplified to 镜.

Korean reading "gyeong." Heavy presence in optical and scientific vocabulary: 望遠鏡 (mangwon-gyeong, "look-far mirror" = telescope), 顯微鏡 (hyeonmigyeong, "make-tiny-visible mirror" = microscope), 眼鏡 (angyeong, "eye-mirror" = glasses), 鏡像 (gyeongsang, mirror image — used in physics and computing), 破鏡重圓 (pagyeong-jungwon, "broken mirror reunited" — a Tang-dynasty literary trope of separated lovers reuniting, the broken halves of a bronze mirror serving as recognition tokens).

Mandarin jìng, 4th tone (simplified 镜). 镜子 (jìngzi, mirror), 眼镜 (yǎnjìng, glasses), 望远镜 (wàngyuǎnjìng, telescope), 照镜子 (zhào jìngzi, "to shine into the mirror" = to look in the mirror). The verb-construction 照 (to shine / illuminate) describes mirror-use specifically — Chinese treats the mirror as something you illuminate yourself in.

Japanese on-reading キョウ (kyō) — 望遠鏡 (bōenkyō, telescope), 顕微鏡 (kenbikyō, microscope). The kun-reading かがみ (kagami) — 鏡 alone, read kagami, means mirror in everyday speech. The compound 手鏡 (tekagami, hand-mirror / pocket mirror) is common. Japanese culture treats the mirror as one of the Three Sacred Treasures (三種の神器) of the imperial regalia, the Yata no Kagami enshrined at Ise. The mirror in Shintō represents purity, truth, and the sun goddess Amaterasu. And the brilliant idiosyncrasy: 眼鏡 ("glasses") is read めがね (megane) in Japanese — neither on-reading nor kun-reading of either character, but a 熟字訓 (jukujikun) where a native Japanese word is mapped onto the Sino-Japanese compound. Megane is the everyday word; ganyō is the formal medical term.

Memory aid: metal (金) polished to completion (竟) — the reflective surface of a bronze mirror.